1. Field of the Invention
The present invention relates to an echo suppressor for application to full-duplex speech communication such as hands-free telephony and voice recognition in a noisy environment. The present invention is particularly useful for applications where the acoustic echo path of a full-duplex speech communication system is severely affected by nonlinear distortion.
2. Description of the Related Art
In a full-duplex speech communication system such as a telephone or a notebook computer operating in a hands-free mode, distant signal from a far-end talker is transmitted from the loudspeaker 2 and some of the acoustic energy is sensed by the microphone 1 (FIG. 1). Acoustic echoes occur as a result of the distant signal from the loudspeaker 2 being coupled through a channel known as acoustic echo path to the microphone. The acoustically coupled distant signal is then coupled into the return path and propagates through the network to the far-end talker, giving an impression of an echo of the talker's voice. In order to cancel the echo, a linear echo canceller 3 is provided. As described in a technical paper “The hands-free telephone problem: an annotated bibliography updated”, Eberhard Hansler, Annals of Telecommunications, 1994, pages 360-367, the linear echo canceller 3 has a replica of the transfer function of the acoustic echo path to produce an echo replica of distant signal. The echo replica is used in a subtractor 4, or residual echo detector to cancel the echo contained in the output of microphone 1, producing an echo-free local signal. A speech detector 5 is provided to monitor the outputs of echo canceller 3 and subtractor 4 as well as the local and distant signals for detecting speech activity of the near-end talker. Speech detector 5 produces a zero or a near-zero output when the near-end speech activity is high and a high-level output when it is low or zero.
The linear echo canceller 3 includes a linear adaptive filter 7. This filter performs a linear filtering on the distant signal and supplies its output to the subtractor 4, while its filter coefficients are constantly updated through a feedback loop according to the output of subtractor 4. The updating algorithm of linear adaptive filter 7 is a process of correlation calculation such that the residual echo at the output of subtractor 4 is reduced to a minimum. As a result, those components of the microphone signal which are correlated with the distant signal are minimized. A multiplier 8 is provided in the feedback loop to prevent near-end speech activity from disturbing the filter coefficients. When the near-end speech activity is high, the output of speech detector 5 is zero or near-zero, which nullifies the multiplier 8 so that the filter coefficients are frozen.
Nonlinearity is of another concern to the design of the echo canceller. The prior art echo cancellation may be satisfactory in so far as the nonlinearity of the acoustic echo path is of small magnitude and the linear echo canceller is able to replicate it. However, in practical systems the operating characteristics of transducer elements of the loudspeaker are far from ideal. Their nonlinear characteristics are of such a magnitude that the linear echo canceller cannot completely replicate the transfer function of nonlinear acoustic echo path. This is particularly true to cellular phones or notebook computers where their small-sized loudspeakers are operated in a high-powered hands-free mode. Due to their severe nonlinear characteristics, acoustic echo cannot be suppressed by more than 20 dB. The remaining echo component would propagate through the network and seriously impede the distant talker.